SPOILER ALERT: Haven’t seen “Prometheus” but you plan to? You might want to read this post later.

From the moment Swedish actress Noomi Rapace signed on to play Elizabeth Shaw in Ridley Scott’s sci-fi action film “Prometheus,” she was concerned that acting in front of a green screen would be a big challenge. But Scott had no intention of putting Rapace on an empty stage. Rather, the director and his crew built enormous sets at Pinewood Studios in London to create the far-off planet where Shaw and her crew land after two years of space travel.

A monolithic figure on a distant planet visited in “Prometheus.” (20th Century Fox)

Even the monsters in Scott’s world were real — including the baby alien that Rapace’s Shaw rips out of her belly with help of a high-tech, robotic surgery machine — the kind every space ship without a doctor aboard should carry. The climatic scene in the middle of the film was shot over four days, a period that Rapace says she’ll always remember as the most stressful of the entire shoot.

The scene, according to Scott, is the one that tipped the film’s rating from a PG-13 to an R. The director said the only way to land the more family-friendly rating would have been to remove the scene entirely.

“They didn’t even want the scene,” Scott said. “It wasn’t about just cutting it down, they didn’t want the scene.”

And that was something neither Scott nor studio chief Tom Rothman wanted considering the importance of the sequence and the toll it took on the Rapace.

“I was in complete psychological meltdown when we were doing those scenes,” said Rapace, 32, who is the mother of a son, Lev, born in 2003. “I felt feverish and I was dreaming these really crazy, dark twisted dreams. It effected me a lot. I was really not well.”

In her dreams, she believed she had her own alien baby growing inside of her–one that she needed Scott to help extract.

“I better call Ridley,” she said, laughing, recalling the dreams. “He was the one I was going to call. What was I going to say to him? I have something in my tummy, come and help me.”

In another dream, she was standing in front of a bathroom mirror watching her skin, her eyes, her whole self turn pitch black. “Then I woke up for real. I was sweating. My bed was wet. I was drained and I really felt like I was losing it.”

It probably didn’t help that during filming, affixed to the top of the machine was the actual alien baby — a heavy, silicone creature being maneuvered remotely off-camera. Despite the creature being utterly repulsive to Repace, the actress still tried in some odd way to connect to it.

“It was weird because I think we as human beings desperately want to create connections,” Rapace said. “So I remember when I saw that thing, my baby, hanging there, I was trying to find something to like about him. And Ridley would come in and say, ‘It’s a cutie, huh?’ Yeah, maybe if you see it from this angle.”

Ridley Scott and Noomi Rapace on the set of “Prometheus” (20th Century Fox)

Despite the other-worldly circumstances, the actress tried hard to make her character feel real and she pushed Scott to add scenes that showed her humanity.

“She’s not an action heroine, a cartoonish superhero — I wanted the audience to feel how hard it is to run around,” Rapace said. “You run out of steam. You can’t do a caesarean on yourself and stay strong. That’s why we put the morphine shots in. I asked Ridley to put in a scene where she cracks, where she can’t take it anymore — that scene where I start to cry in the helmet. It’s too much. For God’s sake, she lost her man, she’s pregnant with something. She takes that thing out of my body. It’s crazy. I really tried to make her real. To make her a real woman.”